The invention relates to an improved process for the production of particularly finely divided milled material, preferably finely divided blowing agent powders having a narrow particle distribution.
Blowing agents are used industrially, inter alia, for the foaming of PVC, rubber, polyolefins, such as polyethylene or polypropylene, and other thermoplastic polymers. The chemical synthesis of azodicarbonamide as one of the most important blowing agents is generally known and is described, for example, in DE-A1-69116867 (U.S. Pat. No. 5,241,117). Today, these blowing agents are used in the form of their finely divided powders, to a lesser extent also in blowing agent formulations as mixtures with activators and/or other blowing agents, and as polymer-specific masterbatches. Depending on the desired application, the blowing agent powders have different particle finenesses, which are obtained by dry milling of the blowing agents after their synthesis and drying.
Customary mechanical milling methods, such as a ball mill, vibratory tube mill, pinned-disc mill or impact mill, are suitable for the dry milling.
Owing to the explosiveness of the products, air-jet mills in the form of spiral jet mills are preferably used, but the disadvantages thereof are a high specific milling energy input and, associated therewith, high milling costs and a broad grain size distribution of the products obtained. In particular, the products milled in this manner still contain so-called “coarse grain”, which, in the context of this Application, is understood as meaning fractions of isolated, unmilled coarse particles, which can lead to problems, in particular foam defects, in subsequent use.
It was therefore the object of the present invention to provide an improved process for the milling of material to be milled, preferably of blowing agents, which requires a low specific milling energy input and after which a product free of coarse grain and having a narrow particle distribution is obtained.